Magnetic recording media having a plurality of magnetic recording layers separated by non-magnetic isolation layers have been shown to exhibit an improvement in some magnetic recording properties, in particular a reduction in media noise, hence an improved signal-to-noise ratio (Hata, et al., 1990a, 1990b; Ranjan, et al., 1991). The reduction in media noise is often attributed to the decrease in thickness of the magnetic layers in such media, since inter-granular exchange coupling is reduced in thinner films (Hata, et al., 1990b; Ranjan, et al., 1991; Sanders, et al., 1989).
However, as the magnetic recording layer decreases in thickness and as the inter-granular exchange coupling is reduced, other magnetic recording properties are compromised. For example, the signal amplitude is reduced in proportion to film thickness (Lambert, et al., 1990), and thinner magnetic recording layers may provide insufficient amplitude of read-back signal because of a low remanence thickness product (Hata, et al., 1990a). Further, a decrease in intergrain exchange coupling is accompanied by a reduction in coercive squareness, S* (Hata, et al., 1990a, 1990b). A consequence of a decrease in coercive squareness is a degradation in overwrite with a larger field required to saturate the medium (Yogi, et al., 1991).
Another factor contributing to degradation in overwrite in magnetic recording media having a multilayer recording structure is that second and subsequent magnetic layers have a progressively lower coercivity than the first magnetic layers, due primarily to the progressively decreasing substrate temperature during the sputtering process (Cord, et al., 1993). The coercivity differential in multilayer media makes saturation of the medium by the read/write head more difficult, causing poor overwrite performance.